Method of transferring impregnated wood chips from an impregnating vessel to a digester



1970 5 A. w. GESSNER 3,525,665

METHOD OF TRANSFERRING IMPREGNATED WOOD CHIPS FROM AN IMPREGNATING VESSEL TO A DIGESTER Filed Oct. 22, 1965 Air or Inert Gas Deaerated 22 Wood GM 5 INVENTOR Adolf W. Gessner BY 77Za/m ATTORNEYS United States Patent F METHOD OF TRANSFERRING IMPREGNATED WOOD CHIPS FROM AN IMPREGNATING VES- SEL TO A DIGESTER Adolf W. Gessner, Montclair, N.J.', assignor to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Oct. 22, 1965, Ser. No. 502,269 Int. Cl. D21c 3/26 US. Cl. 162-19 2 Claims ABSTRACT OF THE DISCLOSURE In the treatment of cellulosic material wherein the material is treated in seriatim in an impregnator, liquordrainer and digester, the digester and liquor-drainer are maintained at about the same pressure, a pressure higher than the impregnator, by introducing a compressed gas into the liquor drainer. In this manner, steam in the digester is prevented from entering the liquor drainer.

This invention relates generally to the treatment of cellulosic material and, more particularly, the invention relates to an improved method of transferring impregnated wood chips and the like from an impregnating vessel to a digseter, and apparatus therefor. The process results in closer thermal and chemical control of both the impregnation and digestion stages of the pulping process.

In pulping processes employing separate chip impregnation and cooking steps, as for example rapid alkaline pulping processes, it is essential that the temperature in each stage be controlled precisely. In such processes the chips or other cellulosic material are treated in seriatim in three stages. First, they are impregnated with white liquor and recycle liquor at a first temperature and pressure in an impregnation zone, the chips are then commonly drained of excess liquor in a chip drainer, and lastly they pass to the digestion zone. The chip drainer may also convey the chips to the digester. The liquor is recycled to the impregnating zone. The digester is operated at higher temperatures and pressures than the impregnating vessel. In continuous vapor-phase pulping close temperature control is extremely difficult, due to the escape of steam at digestion temperature and pres sure from the digestion vessel into the chip drainer. The cooking steam, which is typically at a temperature in the range of about 170 to 185 C., comes into contact with the excess impregnation liquor draining from the chips. As long as there is any temperature difference between the cooking steam and the impregnation liquor, some cooking steam will condense into the liquor, causing its temperature to rise to a temperature approaching that of the steam, and at the same time diluting the impregnation liquor.

Heretofore, the above-described condensation has been considered essentially inescapable. The dilution problem has been overcome by flashing off steam from the hot, diluted impregnating liquor at a lower pressure prior to recycle. While this preserves the liquor at design concentration it does not prevent the loss of high pressure cooking steam. Moreover, it requires the use of an extra stainless steel flash drum, a level controller and a pressure controller.

Accordingly, it is an object of the present invention to provide an improved method and apparatus of transferring cellulosic material from an impregnation zone to a digestion zone which overcomes the deficiencies of prior art methods.

Another object of the present invention is to provide a method and apparatus for transferring impregnated 3,525,665 Patented Aug. 25, 1970 "ice wood chips to a digestion vessel without loss of cooking steam or dilution of the impregnating liquor.

Still another object of the invention is to provide a method and apparatus for transferring impregnated Wood chips to a digestion vessel wherein temperature control of impregnation and digestion is improved.

Various other objects and advantages of the invention will become clear in the course of the following description of an embodiment thereof, and the novel features will be particularly pointed out in connection with the appended claims.

'In accordance with the present invention, impregnated wood chips and excess liquor are fed from the impregnating vessel into a chip drainer, which may be of essentially conventional design. Commonly, this is a screw conveyor having a screen covering a trough in the bottom through which the excess liquor passes. A conduit or conduits and appropriate pumping means return the liquor to the impregnation vessel. The drained chips are dropped from the drainer through an opening and pass into a conduit leading into the digester. It is of course at this point, when the digester feeding valve opens, that cooking steam normally escapes and passes into cooler portions of the system. In the present invention, however, a small amount of high pressure air or inert gas is provided in the discharge end of the drainer. This gas serves to maintain the total pressure in the drainer at the same level as the pressure in the digester. However, the partial pressure of water vapor in the drainer may be much lower than that in the digester, and the impregnation liquor temperature may thus be at any temperature below the cooking temperature. The difference between the partial pressure of water in equilibrium with the impregnation liquor and the total pressure in the drainer is made up by the partial pressure of the added gas in the drainer.

The compressed gas passed into the drainer is vented as required through an opening in the conduit leading to the digester. Any steam which does leak upwardly against the air pressure will also be removed through this opening, so the liquor is never diluted.

A better understanding of the invention will be gained by referring to the accompanying drawing, which is a greatly simplified schematic illustration of the apparatus of the invention, and the following discussion of same.

With reference to the drawing, deaerated wood chips, in line 10, fresh cooking liquor and spent liquor recycled from the digester in line 12, and recycle impregnation liquor in line 16, are all passed to the impregnating vessel 18 in the conventional manner. Compressed air or an inert gas in line 20 may be employed to control pressure in impregnator 18, line 22, with a back pressure control valve, being provided for exhaust.

A suitable pump 24 below impregnating vessel 18 feeds impregnated chips and liquor via line 26 into chip drainer 28. The chips are conveyed along the length of drainer 28 by screw conveyor 30, which is equipped with a screen in the bottom thereof, indicated generally by dotted line 32, through which the liquor passes. A plurality of conduits 34 collect the liquor and return it to liquor recycle line 16.

Compressed gas is supplied to drainer 28, from line 36, which may include a flow indicating controller 38 which opens or closes flow control valve 40 as required. Chips reaching the end of drainer 28, fall into conduit 42 located directly above digester 44, shown only partially. A rotary feed valve 46 may be placed in the top of the digester to pass the chips into the digester, but the invention will work without such a valve. Cooking steam is supplied to digested 44 through line 48. An air vent 50 is located in conduit 42 or in the casing of rotary feed valve 46, these locations assuring that any steam coming from digester 44 will be drawn off before it reaches the cooler chip drainer. Vent S is provided with a suitable back pressure controller and valve 52.

In operation, drainer 28 is maintained at approximately the same temperature as the impregnator and the same pressure as the digester. Thus, the chip slurry entering the drainer via line 26, which is 15% solids by weight, may be at about 120 to 140 C. Impregnating vessel 18 is maintained at only 0.8 to 2.4 atm. gage, however, and immediately on entry into the drainer the chips are subjected to the digester pressure of 511 atm. gage, maintained by compressed gas from line 36. The pressure differential between the drainer and the impregnator reduces the amount of work necessary to recycle the liquor.

As noted above, the compressed gas in line 36 can be air or an inert gas; it is to be noted, however, that air should not be used when the presence of oxygen will adversely afiect the pulp brightness and strength. In such an instance, a gas generated by burning a fuel in air to consume the oxygen may be employed. To completely consume the oxygen, there should be a stoichiornetric excess of fuel. When the cooking liquor is alkaline, CO should :be scrubbed from the combustion gases before injection in line 36 to avoid decreasing the efiective alkalinity of the liquor by absorption of CO and formation of sodium carbonate. Of course, other inert gases can be employed when use of air is restricted. For instance, natural gas (predominently methane) or town gas (predominantly H and CO) or nitrogen are all satisfactory. Since only moderate quantities of gas are required, the cost is not a significant economic burden. By virtue of the foregoingfit will be understood that the expression air or an inert gas is not truly alternative, the choice being dictated by these considerations. Naturally, air will be used whenever possible.

By virtue of the foregoing, steam from the digester is effectively prevented from entering the drainer, temperature control of the impregnating and digester vessel is made easier because the recycle liquor is not heated in the drainer and steam losses from the digester are minimized and, lastly, there is no dilution of the recycle liquor.

It is to be understood that various changes in the details, steps, materials, arrangements of parts, and design of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as defined in the appended claims.

What is claimed is:

1. In a process for treating cellulosic material in seriatim in an impregnating zone, a liquor-draining zone and a digestion zone, and wherein said impregnating zone is maintained at a first temperature and pressure and said digestion zone is maintained at a second, higher temperature and higher pressure, the improvement comprising maintaining the temperature of the liquor draining zone at about the same temperature as the impregnating zone and maintaining saidliquor-draining zone at about the same pressure as said digestion zone by introducing compressed air into said liquor-draining zone.

2. The process as claimed in claim 1 wherein the impregnating zone is maintained at a pressure between about 0.8 and about 2.4 atm. gage and the liquor-draining zone and digestion zone are maintained at a pressure between about 5 and about 11 atm. gage.

References Cited UNITED STATES PATENTS 2,996,422 8/1961 Durant et al 162237 X 3,446,698 5/1969 Shick l62-19 3,259,538 7/l966- Schnyder 16219 FOREIGN PATENTS 463,363 2/1950 Canada.

HOWARD R. CAINE, Primary Examiner US. 01. X.R. 

